Propeller

Controllable Pitch Propeller (I wish)

The benefits of controllable pitch propellers (CPP) specifically
for a sailing/cursing/workboat.

* The props speed vs. power can be adjusted according to varying
cargo loads.
* Drag can be reduced or increased to meet the sailing conditions.
* The pitch can be adjusted match meet the required rpm of a
generator that is being powered from the drive shaft.
* No reversing gear is required.
* Prop walk can be reduced by reducing the pitch.
* With a reversing gear and prop you can prop walk the boat in
either direction.
* Individual blades can be replaced.

Typically, the drop in propeller efficiency owing to the larger
boss (prop hub) size of a controllable pitch propeller is about 2 percent.
However, if a controllable-pitch propeller is well designed and
correctly operated, it can result in fuel savings of up to 15
percent compared with a fixed-pitch propeller operating in a nozzle.
--www.fao.org

The big down side to a controllable pitch prop is the cost. Hundested; www.hundestedpropeller.dk, is renown for some of the
best controllable pitch props so got a quote from McGowan Marine Inc. 1-508 990-1114
stevegow@aol.com, on a new system for a
70ft motor sailor is $66,913 plus shipping. Needless to say I am no longer looking a new Hundested drive
systems.

Our Used Hundested CPP

Paul Liebenberg sold us one of two Hundested controllable pitch
propeller drives he had collected for possible use on his 65 footer.
The price was great, but the problem is that we have to figure out
how to make the blades that are missing. They are no longer in
stock in Denmark and to have them made there will cost us close to
$10,000. But the more we look at it the more it looks like
casting and milling our own blades will be the thing to do. So
the Hundested is getting it's own web page:
Controllable Pitch

Folding or Feathering Props (Still too pricy)

Max-Prop self feathering prop

If you can control the pitch at the push of a button then the
next best thing for a sailboat is to fold or feather the props
blades so the don't slow the boat down when it's under sail power.
Max-Prop; www.max-prop.com
manufactures and sells a self feathering props in a range of blade
configurations. I'd love to have one, but our boat would require a
35", 3 blade propeller and the cost is $15,500. And
being a workboat I doubt it would be very long before I was needing
to replace a blade or two due to damage caused by operator error.

Affordable Fixed Pitch Props (Better)

Buying a Prop

If you live on the coast this will be a piece of cake, but if you
live in Tulsa, Oklahoma you will likely have to shop by email.

Brent Swain's advise "Get a prop within 4 inches of the pitch you
want, preferably closer. Any more and if you try to re-pitch a prop
more than 4 inches the blade breaks off."

To test a used prop is to rest it on the hub and then tap the end
of each blade. It should produce a clear ringing. If it
is dull and short lived then pass it up.

CraigsList turned up more than a few props down toward New
Orleans, so they are out there. Check:
www.propellerplace.com

Another option is a new stainless steel prop from Kahlenberg
propeller that is intended for use on work boats. We plan to
frequently work in shallow waters and close to obstacles so a
stronger prop might be worth the extra money. The price for
a 3 blade 304 stainless steel propeller 32" in diameter with a 27"
pitch, left hand, bored for a 2.5" shaft is $3,995; F.O.B. Two
Rivers, Wi. Contact: Steve Kahlenberg, Kahlenberg Bros. Co., Ph:
920-793-4507, www.kahlenberg.com.

Kevin Morin builds boats up in Alaska and he highly recommended
stainless steel wheels; "My examples were fishing commercially,
justification for these wheels was their ability to run in gravel
bars of Bristol Bay and keep running without damage."

Rotation

Most engines are left hand rotation. Meaning that if you
look at the engine's fly wheel from the back it will rotate
counter clockwise, or the top of the flywheel will be moving toward
the left. Also most inboard props are left hand meaning they push
the boat forward when they spin counter clockwise as seen from the
back end.

Pitch and angle of the blade are not the
same. Consider that the root of the blade closest to the boss
cuts through much less water in one revolution than does the tip of
the blade. So the tip of the blade has a much smaller angle to
the shaft that the root. If both the tip and the root are
pitched the same then both would travel the same distance through
the water when given one rotation.

Building a Steel Propeller from Scratch ( Yeah Baby - Field
Repairable )

I came across a bit below in the Wylo-II group from Lex
Hodgkinson.

"Hi Passepatu has had a mild steel propeller welded to a mild steel
shaft for over 14 years now, I welded up the prop from 5mm (.1967"
3/16=0.1875) plate
scraps and welded this directly on to the shaft. This system has
been pretty much trouble free
with only two cases of mild pitting on the blades when I allowed the
small anode on the prop boss to get eaten away. This was remedied
with an electric weld build up of the pitted areas followed by
grinding, all done in Situ.
(on the hard!) The system has a small anode on the prop boss, a
small one on the rudder and a similar one on the hull close to the
prop shaft area. I cast these myself, they are roughly 1" dia by one
inch long and last around a year.

The propeller is painted with chlorinated rubber paint (as is the
whole boat) and antifouled with the same paint as the hull
(ablating, not the best but I found "hard" antifouling to last even
less) The shaft bearing is "Thordon" or similar, some sort of fibre,
and has never been replaced, the inboard seal is a "dripless" fibre
ring that bears on a bronze plate, which I replaced with stainless
steel. Works ok but the grease filled shaft leaks grease inboard,
just a nuisance. No water leakage at all, just cobwebs in our bilge!
We have no inboard bearing, all thrust being taken by the Hurth
gearbox.

We have a dry exhaust and therefore no cooling water to exit at this
seal and cool it, (I think that was the cause of the bronze plate
wearing too fast) The whole thing works very very well, cost begger
all and seems to incur no problems, I would be far more nervous with
a thousand dollars or more tied up in a machined stainless shaft
with bronze prop. I have a grease gun piped into the prop shaft tube
and give it a squirt now an then.

Incidentally it is easy to alter the pitch if you feel so inclined,
by either belting it with a hammer or twisting the blades with a
large wrench The prop is two blade 17x11 approx and the motor a 35hp
Nannidiesel. It will max out at around 2500 rpm (max design rpm 3000)
Hope this helps, DONT let the "experts" say you cant do this in mild
steel, it works and can be repaired or replaced easily and cheaply,
Incidentall we have been careful not to have anything but mild steel
underwater. Lex"

Lex is using a 1.8 to 1 Hurth gearbox, so the 3000 engine RPM is
1666 at the prop.

"The prop was built using a simple steel jig to compare the blade
shapes to ensure they were the same and at right angles to each
other, pitch was checked at two or three locations with more
attention paid to the outer 1/4 and the tips. Balance was checked
simply by mounting on centers and grinding to achieve no bias. The
blades with some reinforcement at the roots were mounted into slots
cut into a 2"dia hub which was drilled to 1 1/4" to accept the
shaft. The hub is welded to the shaft only at the aft end."
--Lex.

Our prop is 36", not 17" so we got a bit more force involved in
order to pitch the blades. They make special anvil called a
"pitch block" for hammering a prop into shape, but that's for
smaller speed boat props. However a pattern would be really
helpful when bending the blades.

So our 3 blade prop turning at 500 cruising to 719 RPM running at
wide open throttle gives us an frequency from 25 to 36Hz. If we
dropped to 2 blades, that would be 17 to 24Hz.

A prop also have a natural frequency which is mainly derived by the
length and thickness of the blades. If you whack the blade with a
hammer, it will resonate at it's "natural frequency", like a tuning
fork. If the generated frequency from the RPM is close to the props
natural frequency then the amplitude or strength of the vibrations
will greatly increase. Like two kids on either end of a jump rope.
If one shakes the rope up and down fast and the other does it slow
then the rope never bounces that high. But if they get in sync with
each other then the wave in the rope is magnified. Soon the rope
breaks and hits both kids in the head and their parents subsequently
sue the school for not training their kids on how to properly use a
jump rope; but I digress.

So the idea is to separate the frequencies as much as possible. The
natural frequency of the prop is a function measurement of how stiff
the blade is built. A really thick, stiff blade will have a higher
frequency that an thinner, floppier blade. Since a floppy blade is
easier to destroy it makes since to make the blade with a higher
frequency. The problem is that the bigger the blades the lower the
natural frequency. We could make the frequency created by spinning
the prop higher by increase the rpm, but that adversely impacts
power. However a 3 blade prop is better that 2 so that is an easy
choice. And since adding thickness to a blade only slightly affects
it's performance. Doubling the plate at the root of the blade would
likely go a long way increasing it's frequency. It won't do any harm
when it smacks into a log either.

Is that it? I don't guess there is an easy way to predict the
natural frequency of a blade is there?

Stuffing Box / Shaft Seal

PSS Seal

Volvo, Rubber Stuffing Box

PSS Dripless
Shaft Seals are a popular choice but they do leak when shifting
directions if the shaft has any slip in a thrust bearing, and they
are also $440 for a 2" shaft. For me the down side is that the
PSS is not designed for contact with oil or grease and I may want to
use a mild steel driveshaft and protect it by sealing the outboard
end of the shaft log and flooding the log with oil.

Volvo Penta makes a
Rubber Stuffing Box for 2" shafts with 2 3/4” sleeves which sell
for $165 (2010). They say to grease it every 200 hours so it
should be fine to flood the shaft log with oil. The requirement for
a 2 3/4" sleeve is a bit of a problem too as a 2 1/2" pipe has an OD
of 2.88". Water resistant grease is applied to the inside
of the seal by squeezing the sleeve to vent it and then applying the
grease to the inside.

A
Traditional Stuffing Box, drips, and we don't want the drip,
especially if we flood the shaft log with oil to protect the log and
steel shaft.

Prop Resources

Hundested Controllable Pitch Prop

So we have all of the working bits except for the blades.
We could go with stainless steel plate bent to shape and welded onto
lathed stainless bar. Or we can go for a closer proximity to
the original and cast our own from scrap props.

Why Controllable Pitch?

Used Hundested VP-3 FR-H Teardown

The weather is getting colder so we moved the Hundested inside
and starting taking it apart and cleaning it up.

Unfortunately we discovered that their is a collar for the
hydraulic system missing from the controller. The real name from
this is the distributor ring.

For support for Hundested you can actually call Mogens
Christensen in Denmark who now owns the Hundested company. Or
you can call Joel with Pacific Marine Equipment Sales in Seattle at
1-206 281 9841 . I've spoken to both, and both are very
helpful, but Joel understands my Oki accent better. :)

How Ship Propeller Blades are Cast

These illustrations from Marine Propellers and Propulsion show
how propellers were traditionally molded and cast.

I contacted Brain at Boat City Prop Shop in Oklahoma City. And
he'll sell us the Nibral out of his pile of scrap props for scrap
metal prices.

So the next step is to start on a CAD design that approximates
the original blades, but with the diameter and surface area
appropriate for our boat.

Find the difference between the leading and trailing edges of the
blade at a point 2/3 to 3/4 radius from the center of the hub.

Height for leading edge.

Less the height for trailing
edge.

And the angle formed
between the center of the
hub and each of the two
measured points

Formula
Pitch = Difference between the leading and trailing edges of the
blade at a point 2/3 to 3/4 radius from the center of the hub * 360
degrees / the angle formed between the center of the hub and each of
the two measured points.

Pitch Angle for a Desired Pitch

In order to design a prop of Seeker in CAD we need to find the
pitch angle that will create a desired pitch. Pitch Angle is the
angle of the pressure face along the line from the leading edge to
the trailing edge at a given radius with respect to the plane of
rotation measured in degrees. Pitch angle decreases from the blade
root to the tip in order to maintain constant pitch.

If Seeker's prop was fixed pitch, then it should have a pitch around
25 to 27 inches. So we can optimize the blade for 26" pitch
and then the Hundested can adjust it from there. This will
make for a prop that has a twist and therefore it will have more
drag when feathered for sailing, but it will be more efficient when
we are burning diesel, which unlike wind, is not free.

We can then check it using the Manually Measuring Prop Pitch
formula above:

Pitch = (Difference between the leading and trailing edges of the
blade * 360 degrees) / angle formed between the center of the hub
and each of the two measured points.

Pitch = (3.06 * 360) / 40.88 = 26.94 ( close enough for me)

I had to test adding a skin many times before I was satisfied with
the results, but in the end there were only 7 cross-sections.
Each of these cross-sections were the aligned to the desired 26"
pitch using the process outlined above.

Next the skin is applied. The nipple on the top was added
to remove distortion in the skin due to the quick change in shape.
It will be cut away later.

The normal load on each blade is just under 1,300 pounds and the
cross section of the root without any fillet is 3.74 sq in.

CNC Cut Pattern

Hundested Blade Patterns

After four test or or failed attempts we got a pattern cut from
MDF, or fiberboard.

There is some unwanted high spots on the blade due to the BobCAD
software. It did fine where the changes are gradual but it make the
tip thinker than planned. However the MDF sands down easily and a
slightly thinker tip may be a good improvement when doing battle
with sand and gravel.

leave the dross on the melt, when you pout have then push it back
Borax for flux or Boric Acid with Sodium Carbonate 50 50 mix
flux tool to push the flux under the surface
clay-graphite crucible, Or coat a steel crucible with Marcote-7 or
ITC-100
skimmer gate
ceramic filter
www.ransom-randolph.comwww.tower-packing.com

Randupson Portland Cement -- Portland Cement can be and is used as a
bonding agent. Known as the Randupson process, it concerns the use
of 15 and 20 mesh, washed silica sand mixed with 10% cement and
approximately 5% water. Moulds should be air dried for twenty-four
hours and may then be dried out more rapidly. Dry mix for 5
minutes then add the water. The mould is formed around the
pattern and allowed to air dry for 24 hours. Once the pattern
is removed the mould can be dried with hot air. The blade is cast
flat with the boss end making the spure and a vent at the tip. Prior
to pouring the mould is preheated the exhaust from the vent
temperature is 245 degrees F. The gate is located at the
bottom of the boss so as to allow the molten metal to rise gently
into the mould which reduces turbulence and so minimizes oxide
formation.
Casting temperature for NiBrAl is 2048F (1120C) to 2300F
(1260C). Rapid cooling of the casting will improved tensile
properties. Contraction will be 2% in the heavy boss section and
less than 1/5% in the blade tip.

Volume 0.2745674 lbs/cu inch

Nickel Aluminum Bronze; Ni-Al Bz, also called "Nibral" and
frequently miss spelled as "Nibrill", has a melting point of 1913 to
1940F. AA Num: 95800